The surface acoustic wave (SAW) sensor offers advantages in that it is wireless, passive, small and has varying embodiments for different sensor applications. Surface acoustic wave (SAW) sensors are capable of measuring physical, chemical and biological variables and have the ability to operate in harsh environments. In addition, there are a variety of ways of encoding the sensed data information for retrieval. Single sensor systems can typically use a single carrier RF frequency and a simple device embodiment, since tagging is not required. In a multi-sensor environment, it is necessary to both identify the sensor as well as obtain the sensed information. The SAW sensor then becomes both a sensor and a tag and must transmit identification and sensor information simultaneously.
Known SAW devices include delay line and resonator-based oscillators, differential delay lines, and devices utilizing multiple reflective structures. Single sensor systems can typically use a single carrier frequency and a simple coding technique, since tagging is not required. However, there are advantages of using spread spectrum techniques for device interrogation and coding, such as enhanced processing gain and greater interrogation power.
The use of orthogonal frequencies for a wealth of communication and signal processing applications is well known to those skilled in the art. Orthogonal frequencies are often used in an M-ary frequency shift keying (FSK) system. There is a required relationship between the local, or basis set, frequencies and their bandwidths which meets the orthogonality condition. If adjacent time chips have contiguous local stepped frequencies, then a stepped chirp response is obtained. See S. E. Carter and D. C. Malocha, “SAW device implementation of a weighted stepped chirp code signal for direct sequence spread spectrum communication systems”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency control, Vol. 47, July 2000, pp. 967-973.
Orthogonal frequency coding is a spread spectrum coding technique that has been successfully applied to SAW tags and sensors as described in D. Puccio, D. C. Malocha, D. Gallagher, J. Hines, “SAW sensors using orthogonal frequency coding,” Proc. IEEE International Frequency Control Symposium, 2004, pp. 307-310. OFC offers several advantages over single frequency SAW tags and sensors including enhanced processing gain, increased range using chirp interrogation, and improved security. In addition, OFC relies on the use of several frequencies making simultaneous sensing and tagging possible in multiple sensor environments. OFC SAW devices are implemented using reflectors with periodicities chosen to match the chip frequency of interest.
As a result, an OFC SAW device contains a series of reflectors whose center frequencies correspond to the OFC signal of interest. In the case of high reflectivity or narrow bandwidth materials, it is desirable to control the reflection and transmission characteristics of each reflector. The present invention uses weighted reflectors as a method of controlling the reflected and transmitted SAW energy. In addition, arbitrary pulse shapes can be achieved using reflector weighting. Furthermore, pulse shaping can be used inphase shift keying, such as inphase and quadrature channels in minimum shift keying (MSK).